In recent years, substantial advances have been made in the technology for treating wastewater, both domestic and industrial, so that it may be introduced into receiving bodies of water with a minimum of pollution. The requirements for treating wastewater have steadily increased. One of the requirements which is becoming more important is the elimination of nitrogen from wastewater effluent after the effluent has been processed for removing other polluting constituents. The desirability of eliminating nitrogen was set forth in a recent United States Environmental Protection Agency publication entitled "Nitrification and Denitrification Facilities, Wastewater Treatment". In this article which was published in 1973, the Technology Transfer Seminar publication stated in part "with regard to eutrophication of surface water, nitrogen in the fixed form of an ammonium and nitrate ions is considered to be one of the major nutrient supporting blooms of green and non-nitrogen fixing blue-green algae. Nitrogen removal from wastewaters is being requested in some areas and considered in many others. Where discharge is to lakes or reservoirs with significant detention times, seasonal removal will not suffice and performance 365 days per year will be expected." This same article recommends biological systems of nitrification followed by biological denitrification to remove nitrogen from wastewater. The biological systems are complicated by various factors, such as the presence of fragile bacteria, temperature change, cell residence time, carbonaceous B.O.D., pH, carbon source availability and potential repollution by the system. As a follow-up to this 1973 article, a 1974 publication by the United States Environmental Protection Agency entitled "Physical-Chemical Nitrogen Removal-Wastewater Treatment" concludes that all processes now under study for removing nitrogen from waste-water have certain disadvantages, and no single process is considered superior and adapted for general use in removing nitrogen from wastewater. Some of the systems outlined in this 1974 article include physical-chemical systems, such as ion exchange and breakpoint chlorination, which systems are of questionable value due to their excessive cost, control difficulties and possible recontamination by the resulting substances. Another system described in the 1974 article is the ammonia stripping process. Ammonia stripping of ammonia nitrogen from wastewater, has at least some theoretical advantages since this system can treat conventional wastewater treatment equipment effluent. In an ammonia stripping process, wastewater containing dissolved ammonia is passed through a scrubbing tower having air circulating therethrough. The circulated air removes a certain portion of the ammonia which separates from the wastewater effluent. In such a system, the pH of the wastewater effluent is increased to concentrate the amount of nitrogen in the form of ammonia gas within the wastewater as compared to the amount of nitrogen in the form of dissolved ammonium ions. Ammonia stripping does not remove all of the ammonia gas since this gas is highly soluble in the wastewater. The ammonia stripping process has the disadvantage of poor efficiency in cold weather and the potential for scaling problems that may reduce its efficiency and also raises concern over ammonia gas discharge to the atmosphere.
The present invention is adapted to operate in many difficult types of processes containing objectionable quantities of ammonia-nitrogen. Ammonia-nitrogen is present in wastewater as both ammonium ions and ammonia gas. The ratio of the quantity of ammonia gas to ammonium ions is a function of both the pH and temperature of the wastewater or wastewater effluent. As an example, the ratio of ammonia to ammonium ions for a wastewater at approximately 25.degree. C may be expressed by the following equation: ##EQU1##
At higher temperatures, the percentage of ammonia gas increases for any given pH. In a like manner, at lower temperatures, the percentage of ammonia gas decreases for a given pH.
In some ammonia stripping processes, the incoming wastewater effluent, which may have been previously processed to remove other pollutants, is mixed with a substance, such as lime, to create a highly basic liquid. The use of lime has the ancillary advantage of removing phosphates by precipitation. The pH of the liquid is increased to over a pH of 10. As was previously mentioned, the higher the pH, the higher the ratio of ammonia gas to ammonium ions in the wastewater or wastewater effluent. By providing higher concentrations of ammonia gas, the present invention, which will be defined later, can remove a higher percentage of the ammonia gas in order to substantially reduce the total ammonia-nitrogen content of the wastewater.
As further background to the present invention, it is known that ammonia gas may be driven from a liquid contaminated with ammonia if the liquid is boiled by heating the liquid to the boiling temperature. This concept is used in the Keljdahl test for ammonia in a liquid. Such boiling may also be induced by lowering the absolute pressure over the body of liquid containing dissolved ammonia gas to the vapor pressure proportional to the temperature of the contaminated liquid.
Disadvantages of prior systems for removing ammonia gas from wastewater or wastewater effluent are overcome by the present invention which relates to a method and apparatus for vacuum desorption of ammonia gas from wastewater effluent. Throughout this specification wastewater is used to mean any wastewater liquid irrespective of prior treatment.